An axial flow, gas turbine engine typically has a compression section, a combustion section, and a turbine section. An annular flow path for working medium gases extends axially through these sections of the engine. A stator assembly extends about the annular flow path for directing and confining the working medium gases to the flow path.
As the gases are flowed along the flow path, the gases are pressurized in the compression section and flowed to the combustion section. The pressurized gases are burned with fuel In the combustion section to add energy to the gases. The hot, pressurized gases are expanded through the turbine section to produce useful work. A major portion of this work is used as output power, such as for driving a free turbine or developing thrust for an aircraft.
A remaining portion of the work generated by the turbine section is not used for output power. Instead, this portion of the work is used internally of the engine to compress the working medium gases in the compression section. A rotor assembly extends between the turbine section and the compression section to transfer this work from the turbine section to the compression section. The rotor assembly has rotor blades in the turbine section which extend outwardly across the working medium flow path for receiving work from the gases. The rotor blades are angled with respect to the approaching flow to receive work from the gases and to drive the rotor assembly about the axis of rotation.
An outer air seal circumscribes the rotor blades to confine the working medium gases to the flow path. The outer air seal is part of the stator assembly of the engine and typically is formed of a plurality of arcuate seal segments. The stator assembly further includes an engine case, such as an outer case, and a structure for supporting the seal segments of the outer air seal from the outer case. The outer case and the support structure position the seal segments in close proximity to the blades to block the leakage of the gases past the tips of the blades. As a result, the segments are in intimate contact with the hot working medium gases and receive heat from the gases. The segments are cooled to keep the temperature of the segments within acceptable limits.
One example of such a construction is shown in U.S. Pat. No. 3,583,824 issued to Smuland et al. entitled "Temperature Controlled Shroud and Shroud Support". Smuland employees on an outer air seal which is adapted by an upstream flange or hook 44 and a downstream hook 46 to engage a support. Cooling air is flowed in a cavity which extends circumferentially about the outer air seal between the outer air seal and an engine case. A seal means, such as an impingement plate or baffle, extends circumferentially about the outer air seal to define an impingement air cavity 58 therebetween. A plurality of holes extend through the impingement plate to precisely meter and direct the flow of cooling air through the impingment plate across the compartment 58 and against the outer surface 59 of the seal segment. The air is then gathered in the impingement air cavity. The cooling air is exhausted from the impingement air cavity through a plurality of axial passages 66 in the downstream hook 46 to provide a continuous flow of fluid through the plate and across the improvement cavity. This cooling air provides convective cooling to the edge region of the outer air seal as its passes through the compartment 64.
Cooling air holes in the inwardly extending hook, such as the holes 66, are not needed for some configurations. One example is an outer air seal formed of seal segments having a metallic substrate and a ceramic facing material. The ceramic facing material is attached to the metallic substrate and bounds the working medium flow path. The circumferential continuity of the hook is interrupted with a plurality of slots to decrease the hoop strength of the hook and to decrease the local variation in stresses in the metallic substrate which results from the presence of the hook. These slots more than adequately vent the impingement air cavity.
However, venting the cooling air from the cavity is not the only concern. In modern gas turbine engines it is also desirable to meter the flow of cooling air from the impingement cavity after it has been impinged against the outer air seal. The second metering provides a tighter control on the use of cooling air. This tigher control is important because the use of cooling air decreases the operating efficiency of the engine. This decrease occurs because the work diverted to pressurizing the cooling air is diverted from the work available for output power.
Because of the plurality of slots which extend through the hook, it is not possible to rely on holes through the hook to meter the flow of cooling air from the impingement air cavity unless the holes extend into the metallic substrate and a seal member is disposed adjacent to the hook to block the flow of cooling air through the slots in the hook. An alternate approach is to use holes in the seal element to provide the metering function. However, it is not always desirable to try to precisely meter the flow of cooling air from the impingement air cavity by either using a seal element having metering holes or using a seal element without holes and to provide metering holes in the substrate.
In addition, it is desirable to more effectively use the cooling air so that increased cooling is provided with the same amount of cooling air or the same amount of cooling is provided with a decrease in the amount of cooling air. More effectively using the cooling air increases output power and increases the overall engine efficiency while still providing enough cooling air so that the outer air seal has a satisfactory service life.
Accordingly, scientists and engineers are seeking to more efficiently supply cooling air to components such as outer air seal segments by both metering the flow of cooling air and more effectively using the cooling air.